Power amplifiers for use in driving, for example, electro-dynamic shakers must be capable of stable operation with a wide range of electrical loads, offer low distortion, make efficient use of electrical power, be compact in design and allow for computer control.
The use of switch-mode power technology in such amplifiers provides many advantages over linear amplification techniques and are available presently in a range of 5 to 300 kVA. The use of switch-mode power technology dramatically improves amplifier efficiency over linear amplifiers, thus reducing power losses. This allows air cooling of semi-conductor heatsinks, as opposed to water cooling. A reduced output semi-conductor component count for switch-mode designs increases reliability also. By using computer operation and control of switching modulations, lower total harmonic distortion figures may be established.
A standard switch-mode amplifier approach is to use a single half-bridge at high frequency (for examples 150 kHz). To reduce distortion from the amplifier a feedback path is applied. The amount of feedback which can be applied however is limited by the switching frequency of the individual switching elements. To obtain lower distortion a higher switching frequency should be used as this allows more feedback. However, in the half-bridge topology, as the switching frequency increases the losses in the power switches increase. This reduces efficiency. Specialised high frequency switching elements may be used but these are inherently expensive. Thus, a compromise is established between the cost of the high frequency switching elements allowable distortion and the output power.
It is an objective of the present invention to alleviate the disadvantages associated with prior art amplification techniques and to provide a switch-mode power amplifier having a controllable high frequency output.